High yield spray application

ABSTRACT

Method and device for ensuring high yields during spraying of a settable slurry composition. The slurry is conveyed through a length of hose to a nozzle spray-orifice, and a set accelerator is introduced into the slurry at a distance “D” from the nozzle spray-orifice. The level of accelerator is monitored using a sensor located in the slurry path at a point after the set accelerator is injected. A preferred sensor comprises two pairs of conductivity sensors each having an annular body with a bore corresponding to the diameter of the hose or nozzle bore. Voltage of the slurry can be correlated to yields obtained when a salt-based accelerator fluid is injected into the slurry, such that levels of accelerator can be finely adjusted, and consistently high yields of spray coatings can be achieved.

FIELD OF THE INVENTION

The invention relates to spray application of rapid-settingcompositions, and more particularly to a method and device for ensuringhigh yield spray application of a settable composition through accuratemonitoring of the level of a set accelerating agent introduced into thespray composition.

BACKGROUND OF THE INVENTION

Spray applications are known which involve the use of an acceleratingagent introduced into a composition after conveyance through a hose. Inparticular, the composition being pumped through the hose is ahydratable cementitious particle slurry or suspension which is expelledthrough a spray nozzle onto a substrate. For example, such acceleratingagents are used in the spray-application of concrete (e.g., “shotcrete”)and fire protection coatings for metal substrates such as steel beamsand panels.

A particularly successful type of product in the fire protection fieldis a gypsum-based slurry formulation that contains, in addition to thegypsum binder, a lightweight inorganic aggregate, such as shreddedexpanded polystyrene, vermiculite, and/or cellulose, along with an airentrainer and other agents. Fireproofing mixes such as the MONOKOTE®brand fireproofing from Grace Construction Products, Cambridge, Mass.,are sold as a dry powder. The powder is mixed with water at the job siteusing a mixer that is situated at ground level. The fireproofing slurryis then pumped through a long hose which sometimes extends to a location20 to 30 stories above ground level. It is sprayed through a nozzle ontoa steel beam, panel, or other construction surface.

As explained by Hilton et al. in U.S. Pat. No. 4,904,503, the slurriesmust possess a number of important properties for spray application.They must adhere to the metal substrate in both the wet slurry state andafter setting. They must set without undue expansion or shrinkage inorder to avoid crack formations that could undermine the insulativeproperties of the coating. The slurry compositions must be able to holda relatively large quantity of water so that they can be pumped easilyand to great heights. Yet, the slurries must retain a consistency so asto avoid segregation or settling of ingredients.

From an economic viewpoint, it is important that the sprayable slurryprovide an adequate “yield.” The term “yield” means the volume ofapplied fireproofing composition, after setting, per given weight of drymix used to prepare the fireproofing composition slurry.

To increase yield, Hilton et al. disclosed in U.S. Pat. No. 4,904,503that an acidic set accelerator could be injected into the cementitiousslurry to react with a basic material contained therein. The reactionprovides carbon dioxide gas for foaming and expanding the slurrycomposition, thereby providing a substantial increase in yield of theslurry after setting. In the '503 patent, it was suggested that theaccelerator be introduced into the slurry close to the nozzle tominimize the potential for hardening of the accelerated mix in the feedline during pumping.

The use of an accelerator has been known to convert the setting time ofthe slurry mix from about 4 to 12 hours to less than about 15 minutes,and it is often preferred to employ an amount of accelerator sufficientto obtain a setting time of about 5 to 10 minutes and also to obtain ayield increase in the range of 0.1% to 20%.

Volumetric flow metering techniques have been used in the sprayapplication of gypsum-based fireproofing as well as sprayed concrete(shotcrete) to control the amount of set accelerator introduced into theslurry. For example, it is believed that flow meters have been used forintroducing an alum accelerator into the hose during thespray-application of gypsum-based fireproofing. As another example, avolumetric flow metering systems are commercially available from NormetCorporation under the trade name Nordoser 900™ and from MBT Pty Ltd ofAustralia under the trade name Viper Roboshot MkIII for use in sprayedconcrete applications.

The present inventors, however, believe that the use of flow meters donot necessarily guarantee accurate monitoring where lengthy hoses arerequired for application of the slurry onto substrates. A sufficientlevel of accuracy is especially harder to attain where the distancesbetween the mixer and nozzle are great. For example, a high-riseconstruction site requires several hundred feet of slurry hose to beemployed. This means that several hundred feet of small diameter tubingwill also be required for pumping the set accelerator to or near thespray nozzle. Accordingly, the hose path from the mixer at ground levelto the spray nozzle may span the distance of 10 to 30 stories or moreabove ground level. The path usually involves winding and circuitousroutes around several obstacles and sharp edges. Typical obstacles forthe accelerator tubing include door and wall openings, stairwells, andmetal studs. The tubing is susceptible to pinching, kinking, collapsing,leaking, and other conditions that prevent a controlled introduction ofset accelerating agent into the slurry.

An objective of the present invention, therefore, is to provide a methodand device for accelerator injection and monitoring that provideaccurate monitoring of accelerator levels actually introduced into theslurry.

Another objective is to provide an accelerator monitoring method anddevice that can withstand the corrosive effect of cementitiouscompositions on metering equipment and pluggage effect on theaccelerator injector; that employ cleaning methods using current linecleaning methods; and that involve un-obtrusiveness to the sprayoperation.

A still further objective is to provide a device that can withstand thehazards of the industrial work environment. For example, the acceleratormonitoring device should be able to withstand repeated drops onto hardsurfaces without any loss in accuracy or effectiveness.

Finally, an important objective of the present invention is to provide amethod and device for obtaining consistently high yield sprayapplication of a settable composition using a set accelerator agent thatis injected into the hose-conveyed composition being spray applied.

SUMMARY OF THE INVENTION

The present invention provides a method and device for achievingconsistently high yield spray application of settable slurrycompositions.

A exemplary method comprises conveying a settable slurry through alength of hose to a nozzle spray-orifice; introducing a set acceleratingagent into said the slurry at a distance “D” from the nozzlespray-orifice; and monitoring the level of set accelerating agent in theslurry, at a distance less than “D” from said nozzle spray-orifice, byemploying a sensor to detect changes in a physical property of theslurry corresponding to levels of set accelerating agent.

The set accelerating agent (or “accelerator”) may be introduced into theslurry by known means. In fireproofing spray applications, an injectionport may be used for injecting the accelerator pumped under pressureinto the hose or into the nozzle.

For fireproofing spray applications, the accelerator is preferablyinjected into the slurry as a water-solubilized salt, and the sensor ispreferably an electrical conductivity sensor operative, in combinationwith a voltage meter or other device, to provide an indication of theconductivity of the slurry. An exemplary conductivity sensor comprises afirst pair of electrodes, preferably having an annular body (such as ahollow cylinder shape), which are operative to generate a voltage fieldwhen attached to a voltage source, and a second pair of electrodes, eachalso preferably having an annular body, located between the firstelectrode pair, are operative to detecting voltage differences betweenthe first electrode pair when the second electrode pair is connected toa volt meter or other measuring device.

The sensor can be installed in the hose or nozzle at a distance “D”e.g., six inches to one hundred feet) from the nozzle spray-orifice at apoint after the injection port through which the accelerator is injectedinto the hose or nozzle. Preferably, the sensor is housed in a sleevehousing at a distance which is at least five feet, and more preferablyat least ten to seventy-five feet from the nozzle spray-orifice, so asto minimize opportunities for damaging the sensor mechanism.

The afore-mentioned conductivity electrodes preferably have inner borediameters that coincide with the hose or nozzle inner diameter tominimize build-up of slurry material on electrode surfaces.

The use of electrodes and metering device allows a correlation betweenslurry conductivity and yield to be made based on actual field testingresults. Accordingly, the present invention facilitates automatic ormanual adjustment of accelerator injection rates into the slurry asconductivity measurements fall below or rise above a predeterminedlevel. In further methods and devices of the invention, visual and/oraudible alarms can be used to indicate when levels of accelerator aredeviating from pre-determined levels or require critical adjustment.

In this manner, the present invention is believed to provide moreconsistently high yield spray-applications, because monitoring andcontrol of accelerator levels is achieved by using a sensor whichprovides an indication of the actual levels within the hose or nozzle,and this is believed to be much more accurate than prior art volumetricmethods which merely monitor the flow rate of accelerator being pumpedthrough the tubing on its way to the hose or nozzle. In other words,accuracy in the present invention is not hindered by blocked, kinked, orleaking tubing or connectors.

The present invention also provides a settable slurry spray applicationsystem and device that are accurate, durable, convenient, andeconomical. The above-described sensors may be housed in a rigid sleevemade of dielectric material, such as polyethylene or polyamide, whichcan be screwed and/or clamped between hose sections, between hose andnozzle; or the sensors may be incorporated directly into the nozzleitself.

Further advantages, benefits, and features of the present invention aredescribed in further detail hereinafter.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic drawing of an exemplary spray application of thepresent invention for settable slurry compositions; and

FIG. 2 is a cross-sectional diagram of an exemplary sensor of theinvention for sensing the levels of set accelerator introduced into ahose-conveyed settable slurry composition.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As used herein, the term “settable slurry composition” means and refersto compositions comprising an inorganic binder material, which, uponmixing with water, form a hydratable slurry that can be pumped throughthe length of a hose and spray-applied by nozzle onto a substrate.

The term “yield” means the volume of spray-applied slurry compostion,after setting, per given weight of dry inorganic binder material used toprepare the settable slurry composition.

Typical inorganic binder materials contemplated for use in the settableslurry compositions include Plaster of Paris, stucco, gypsum, Portlandcement, aluminous cement (e.g., a calcium sulphoaluminate cement, a highalumina cement), pozzolanic cement (e.g., finely ground blast furnaceslag or fly ash, silica fume), gunite, magnesium oxychloride, magnesiumoxysulfate, or mixtures thereof. Suitable settable slurry compositionsare disclosed, for example, in U.S. Pat. No. 4,751,024 of Shu et al.;U.S. Pat. No. 4,904,503 of Hilton; U.S. Pat. No. 5,034,160 of Kindt etal.; U.S. Pat. No. 5,340,612 and U.S. Pat. No. 5,401,538 of Perito; U.S.Pat. No. 5,520,332 of Gaidis et al.; U.S. Pat. No. 5,556,578 ofBerneburg et al.; U.S. Pat. No. 6,162,288 of Kindt et al.; and WO03/060018 of Hilton, all incorporated by reference herein.

A preferred settable slurry composition is sold by W. R. Grace &Co.-Conn., Cambridge, Mass., under the MONOKOTE® tradename. Such asprayable fireproofing slurry composition comprises shredded expandedpolystyrene, as well as other components, such as known set retardingagents (See e.g., U.S. Pat. No. 6,162,288 of Kindt et al.).

A wide variety of alternative aggregate and filler materials may beemployed within the settable slurry. These include exfoliatedvermiculite, expanded perlite, diatomaceous earth, a refractory fillersuch as alumina or grog or colloidal silica, ceramic fibers, mineralfibers, glass fibers, common mixed paper waste, paper mill sludge, pulp,cellulose and the like, or agricultural fibers such as fibers extractedfrom wattle bark, palm fiber, kenaf, reeds, and natural organicparticles such as ground cork and sawdust.

Fibers may comprise dry synthetic particles or fibers such as organicparticles derived from milled thermoplastic foams, for example, phenolformaldehyde resole resin foams, urea formaldehyde foams, andpolyurethane rigid or flexible foams; and organic fibers such as carbon,aramid, polyacrylonitrile, polyvinyl alcohol, polyethylene,polypropylene, polyester, acrylics, and mixtures thereof.

Exemplary set accelerators useful for accelerating the setting of slurrycompositions of the present invention may include those conventionallyknown. As described in U.S. Pat. No. 5,520,332 of Gaidis et al., ownedby the common assignee hereof, set accelerators are preferably lowviscosity fluids which are injected into the slurry to decrease its settime upon a substrate. Acidic set accelerating agents capable ofsatisfactorily offsetting the retardation of the slurry can be used. Formost commercial applications, the type and amount of accelerator is thatwhich rapidly converts the setting time from about 4 to 12 hours toabout 5 to 10 minutes. The amount required to provide such setting timeswill vary depending on the accelerator and the type and amount ofretarder and binder. Generally, an amount in the range of about 0.1% to20% by weight of dry accelerator based upon the weight of drycementiitous binder is used, with about 2% being preferred. Examples ofuseful accelerators include aluminum sulfate, aluminum nitrate, ferricnitrate, ferric sulfate, ferric chloride ferrous sulfate, potassiumsulfate, sulfuric acid, and acetic acid, with aluminum sulfate beingpreferred.

Conventional set accelerator injector ports 16 or mechanisms may beemployed for introducing low viscosity liquid accelerators into lowviscosity liquids, especially those employed in the fireprotection sprayapplication industry and shotcrete industries. Suitable mechanisms havebeen disclosed, for example, in U.S. Pat. No. 4,934,596 of Hilton et al.and U.S. Pat. No. 5,520,332 of Gaidis et al., incorporated herein. Inthe '332 patent, Gaidis et al. disclosed a check-valve-injector portwhich comprises a tube have a plurality of slits along its length whichwas inserted into the slurry path. The tube is made of a stretchablematerial which allows the tube to expand so that the slits pop open. Theaccelerator is injected into the slurry path under pressure through theopened slits. If there is a surge in the slurry pressure, the slits canclose, thereby preventing infiltration of slurry which could block thetube. Other set accelerator agent injector designs may also be employedfor introducing the accelerator into the slurry path within the hose ornozzle.

For the purpose of generating gas or foam within the slurry, it isdesirable to employ a basic material. The term “basic material” refersto any material which reacts with the acidic set accelerating agent usedin the compositions with resultant evolution of a gas and volumeexpansion of the slurry. Preferably, this basic material is added to theslurry composition and is not naturally occurring in the cementitiousbinder. Exemplary basic materials that can be added to the slurry binderto generate gas when combined with the set accelerator includecarbonates such as calcium carbonate, sodium carbonate, sodiumbicarbonate, or mixtures thereof.

As shown in FIG. 1, a mixer 10 is used for combining water and acementitious binder 12 to provide a pumpable and settable slurry whichis conveyed through the length of a hose 14 towards a nozzle having aspray-orifice, as generally designated at 22. Typically the nozzle isformed of a metal, such as aluminum, steel, or other rigid material.Pressurized air is pumped through an air hose 24 to the nozzle andthrough the nozzle spray-orifice 22 by a powered air pump 26 to propelthe slurry from the nozzle onto a substrate (such as a steel beam orpanel, not shown).

In a preferred embodiment of the present invention, an accelerator (thetank of accelerator and pump both being designated at 18) is pumped intothe hose 14 through an accelerator injection port designed at 16 locatedalong the hose at a distance “D” from the nozzle spray-orifice 22.Although the accelerator injection port may be on the nozzle or inproximity to the nozzle, more preferably the distance “D” between theaccelerator injection port 16 and nozzle spray-orifice 24 is no lessthan ten feet and no greater than one hundred feet 22; and morepreferably it is located fifteen to seventy-five feet from the nozzlespray-orifice 22 to allow the acidic set accelerator injected into theslurry (through injection port 16) to react with basic materialcontained in the slurry and to generate gas that will increase the yieldof the slurry when sprayed and dried upon the substrate.

A sensor 20 is employed to monitor the actual amount of set acceleratorintroduced into the slurry, at a point designated at “20” which islocated at a distance less than “D” from the nozzle spray-orifice 22.The sensor 20 is preferably electrically or electronically connected toa power source 32 (which may be a direct current or alternating currentsource) and metering device 38. The power source 32 and metering device38 may be conveniently located near the operator of the mixer 10 topermit automatic or manual adjustments to be made at the pump 18 whichfeeds accelerator into the slurry.

Where the set accelerating agent introduced into the slurry is awater-soluble salt, the sensor 20 can be a conductivity sensor that isoperative to provide an indication of the conductivity of the aqueousslurry. As shown in FIG. 2, an exemplary sensor 20 comprises aconductivity sensor operative to provide an indication of theconductivity of the slurry at said distance less than “D” from thenozzle spray-orifice. Preferably, the sensor 20 comprises an assembly ofelectrodes which are physically isolated from each other and mountedwithin a hose, housing sleeve, or nozzle so as to come into electricalcontact with slurry. The material used for construction of theelectrodes can be any electrically conductive material. 316 StainlessSteel has been commonly used to manufacture the electrodes.

As shown in FIG. 2, the sensor 20 assembly comprises a first pair ofelectrodes 30, which are spaced apart and aligned with the bore of thehose, housing sleeve, or nozzle. These are operative to generate avoltage field within a portion of the slurry when a voltage source 32 isconnected to this first electrode pair 30. A second pair of electrodes34 is also spaced apart but located between the two first pairelectrodes 30, and are connected to a meter 36 or other conductivitydetection device which is operative to detect voltage differencesbetween the first pair of electrodes 30. A portion of the hose orseparate housing sleeve is generally designated as at 14 and is used tophysically and electrically isolate the electrodes 30/34 from each otherand from the external environment.

The electrodes 30 and 34 preferably each have an annular body(preferably a hollow cylinder shape) with a bore aligned with andsimilar diameter with the bore of the hose 14 (or nozzle if situated inor in proximity to the nozzle). While electrode shapes such as stripsand rectangles can be used as an alternative to an annular, the annularbody shape 30/34 is preferred because some portion of the electrodesurfaces must come into electrical contact with slurry thereby providinga more reliable conductivity level reading. In addition, an annularshape that is aligned with the bore (no protruding surfaces relative tothe surrounding surfaces) will prevent slurry material from accumulatingagainst any protruding electrode surfaces.

Hence, an exemplary method for spray-applying a settable slurry,comprises: conveying a settable slurry through a length of hose 14 to anozzle spray-orifice 22 from which the conveyed slurry is spray-appliedonto a substrate (not shown); introducing a set accelerating agent 16into the slurry at a distance “D” from the nozzle spray-orifice 22; andmonitoring 22 the level of set accelerating agent in the slurry, at adistance less than “D” from the nozzle spray-orifice, by employing asensor 20 to detect changes in a physical property of the slurrycorresponding to levels of set accelerating agent being introduced intosaid conveyed slurry.

Preferred devices and methods of the invention comprise the use of asensor 20 in the hose 14 at a point in the length of a hose locatedbetween the set accelerator injection point 16 and the nozzlespray-orifice 22. Preferably, the sensor 20 comprises a first pairconductivity electrodes 30 which are spaced apart a distance from eachother. These electrodes 30 are operative to generate a voltage fieldthrough slurry in the area between the electrodes when a power source 32is electrically connected to them. A second pair of conductivityelectrodes 34 are spaced apart a distance from each other and locatedbetween said first conductivity electrode pair 30. This second pair ofelectrodes 34 is operative to detect voltage differences between thefirst conductivity electrode pair 30 when a voltage meter 38 or otherdetector device is connected to this second electrode pair 34.

In further exemplary embodiments, a voltage meter 38 may be connected towarning lights and/or alarms to provide indication to the operator whenthe conductivity drops below or rises above a predetermined limit, andmay be used to control automatically the injection of set accelerator.

In further exemplary devices and methods of the invention, at least onetemperature sensor 36 may be employed in an area of the hose 14operative to measure the temperature of the slurry in the neighborhoodof the conductivity sensor electrodes 30/34. As shown in FIG. 2, atemperature sensor 36 may also comprise an annular body with a bore thatcoincides with the inner bore diameter of the hose 14. As electricalconductivity of the slurry can be affected by its temperature, the useof a temperature sensor 36 permits temperature deviations to be takeninto account, thus providing greater accuracy in conductivity readings.

The conductivity electrodes 30 and 34, as well as the temperature sensor36, are preferably mounted and embedded within a sleeve housing which ispreferably made of a non-conductive material such as polyethylene,polyamide, or other electrically non-conductive material. The electrodes30/34 and temperature sensor 36 are thus protected from potential damagecaused by slurry conveyed through the hose 14. As mentioned above, theinner bore diameter of the electrodes and the sleeve housing withinwhich they are mounted is preferably the same as that of the hose 14,which can be attached by a screw, clamp, or other known mechanism to thesleeve housing. The outer circumferential diameter of the sleevehousing, of course, may be larger than the outer diameter of the hose asmay be necessary to provide structural integrity to resist internalpressure of the pumped slurry as well as external damage due todropping.

An operator will operate the spray equipment until a desired yield levelis achieved by the slurry when spray-applied and set upon a substratesurface such as a steel beam or panel. The yield measurement ofcommercial fireproofing slurries, such as GRACE's MONOKOTE product, istypically done by measuring cup weight a known volume of slurry exitingfrom the nozzle spray-orifice. When a desired cup weight yield (i.e.,density) is obtained at the nozzle for a given level of set acceleratorintroduced into the hose (via accelerator injector port designated as at16), slurry conductivity as determined by voltage meter 38 can becorrelated with a desired yield.

During an actual application of the slurry at a job site, it ispreferable to locate the meter 38 (as shown in FIG. 2) next to the mixer10 (FIG. 1) so that the mixer operator can make adjustments to theaccelerator pump 18.

In further exemplary embodiments, the metering device 38 can beconnected to visual and/or audible alarms to provide the operator of thespray nozzle 22 or the operator the mixer 10 an indication of the slurryconductivity level, and hence accelerator level, in the slurry. Forexample, when the meter 38 is manually set to the conductivity of theslurry that corresponds to a desired yield value (and hence desirableset accelerator level), an indicator which may be in the form of visiblelight, alphanumeric display, audible alarm or vibratory sensor (such asa paging device) may be used to indicate that an acceptable orunacceptable process condition exists. If the conductivity of the slurryrises above or falls below a predetermined level, indicating that apossible process deviation exists, an indication would be provided tothe appropriate person that the set accelerator level should be checked.If the conductivity of the slurry is above or beyond a predeterminedcritical level, indicating that a process deviation condition existsthat must be corrected, a revised indication would be provided.

In still further exemplary embodiments of the invention, the process canbe automated through the use of a computer processing unit (“CPU”), suchas a lap top computer of a programmable logic controller (PLC). Forexample, the metering device 38 and accelerator pump 18 can both beconnected to a CPU or PLC such that the rate of set accelerator pumped18 into slurry (via injection port 16) can be correlated and adjustedautomatically depending upon the sensor 20 information fed back to themetering device 38.

In still further exemplary embodiments, the sensor 20 may be a pH sensorwhich is operative to detect levels of acidic set accelerator injectedinto the hose 16. Other sensors may be employed, such as ultrasonic,optical, and capacitive sensors. However, the use of electricalconductivity sensors is most preferred for reasons of sensitivity androbustness, while pH sensors were next in order of preference for thesereasons.

As will be evident from the description of the foregoing methods, thepresent invention provides an apparatus for spray-application ofsettable compositions, comprising a hose 14 for conveying a settableslurry to a nozzle spray-orifice 22, a set accelerator injection port 16located along said hose 14 (or at or near the nozzle) at a distance “D”from the nozzle spray-orifice 22, and a sensor 20 located within theconveyed slurry path at a distance less than “D” from the nozzlespray-orifice 22, the sensor 20 operative to provide an indicationcorrelating with the level of set accelerator introduced into theconveyed slurry.

Preferred devices of the present invention for achieving consistentlyhigh yields of spray-applied settable slurry composition comprise aplurality of conductivity electrodes 30 and 34 each having annularbodies, as shown in the cross-sectional perspective of FIG. 2, containedwithin a housing sleeve (14) which could be the hose itself or, morepreferably, a rigid housing sleeve made of non-conductive material suchas polyethylene, polyamide, or other electrically non-conductivematerial. As previously explained, a preferable arrangement is to have afirst pair of field electrodes 30 for generating an electric field whenthe electrodes 30 are connected to a voltage source 32′ and a secondpair of electrodes 34 located between the first electrode pair 30 andalong the slurry path for measuring the voltage difference in the slurrywhen the second electrode pair 34 is connected to a voltage meter 38 orother measurement device.

The foregoing examples are provided for illustrative purposes only andare not intended to limit the scope of the invention.

1. A method for spray-applying a settable slurry, comprising: conveyinga settable slurry through a length of hose to a nozzle spray-orifice;introducing a set accelerating agent into said conveyed slurry at adistance “D” from said nozzle spray-orifice; and monitoring the level ofset accelerating agent introduced into said slurry, at a distance lessthan “D” from said nozzle spray-orifice, by employing a sensor to detectchanges in a physical property of said slurry corresponding to levels ofset accelerating agent introduced into said slurry.
 2. The method ofclaim 1 wherein said slurry comprises Plaster of Paris, stucco, gypsum,Portland cement, aluminous cement, pozzolanic cement, gunite, ormixtures thereof.
 3. The method of claim 2 wherein said slurry comprisesgypsum.
 4. The method of claim 2 wherein said slurry further comprisesan aggregate.
 5. The method of claim 1 wherein said set acceleratingagent is a water-soluble salt, and said sensor is a conductivity sensoroperative to provide an indication of the conductivity of said slurry atsaid distance less than “D” from said nozzle spray-orifice.
 6. Themethod of claim 5 wherein said distance “D” is no less than five feetand no greater than 100 feet from said nozzle spray-orifice.
 7. Themethod of claim 6 wherein said set accelerating agent is selected fromthe group consisting of aluminum sulfate, aluminum nitrate, ferricnitrate, ferric sulfate, ferric chloride ferrous sulfate, potassiumsulfate, sulfuric acid, and acetic acid.
 8. The method of claim 1wherein said set accelerating agent is aluminum sulfate; wherein saidslurry is gypsum, Portland cement, or a mixture thereof; and whereinsaid slurry further comprises a basic material which, upon contact withsaid set accelerating agent, generates a foam within said slurry.
 9. Themethod of claim 1 wherein said sensor comprises an assembly ofelectrodes in physical contact with said slurry, said electrode assemblycomprising a first pair of electrodes operative to generate a voltagefield within a portion of said slurry when a voltage source is connectedto said first electrode pair, and a second pair of electrodes operativeto detect voltage differences between said first pair of electrodes whena voltage meter is connected to said second electrode pair.
 10. Themethod of claim 9 wherein said electrodes each have an annular body witha bore aligned with the hose or nozzle inner diameter.
 11. The method ofclaim 10 wherein said annular electrodes are mounted within a sleevehousing operative to isolate electrically said annular electrodes fromeach other.
 12. The method of claim 9 further comprising at least onetemperature sensor operative to measure the temperature of slurry in anarea proximate to said sensor.
 13. The method of claim 1 wherein saidsensor is a conductivity sensor comprising a plurality of electrodesoperative to provide an indication correlating with the electricalconductivity of slurry, said electrodes being connected to a meter; saidmethod further comprising adjusting the level of set accelerating agentintroduced into said slurry in response to said indication correlatingwith the electrical conductivity of the slurry at a point after saidaccelerating agent is introduced into said slurry.
 14. The method ofclaim 13 wherein said indication is a visible light, alphanumericindication, audible alarm, vibratory device, or combination thereof. 15.A method for spray applying a settable composition, comprising:conveying a settable cementitious slurry comprising gypsum, Portlandcement, or mixture thereof through a length of hose to a nozzlespray-orifice from which said conveyed cementitious slurry isspray-applied onto a substrate, said slurry further comprising at leastone aggregate and a basic material; introducing an acidic setaccelerating agent into said hose-conveyed slurry at a distance “D” fromsaid nozzle spray-orifice, said acidic set accelerating agent operativeto react with said basic material contained within said slurry and toincrease the yield thereof when spray-applied through said nozzlespray-orifice onto a substrate; and monitoring the level of said acidicset accelerating agent in said conveyed slurry, at a distance less than“D” from said nozzle spray-orifice, by employing a sensor to detectchanges in a physical property of said slurry corresponding to levels ofset accelerating agent being introduced into said slurry, said sensorcomprising an assembly of electrodes in physical contact with saidslurry, said electrode assembly comprising a first pair of electrodesoperative to generate a voltage field within a portion of said slurrywhen a voltage source is connected to said first electrode pair, and asecond pair of electrodes operative to detect voltage differencesbetween said first pair of electrodes when a voltage meter is connectedto said second electrode pair.
 16. The method of claim 15 furthercomprising adjusting the level of set accelerating agent introduced intosaid slurry in response to an indication provided by said voltage meter.17. A spray-application device, comprising a hose for conveying asettable slurry to a nozzle spray-orifice, a set accelerator injectionport located along said hose at a distance “D” from said nozzlespray-orifice, and a sensor located along said hose at a distance lessthan “D” from said nozzle spray-orifice, said sensor operative toprovide an indication correlating with the level of set acceleratorintroduced into the slurry.